A biomimetic laminated strategy enabled strain-interference free and durable flexible thermistor electronics

The development of flexible thermistor epidermal electronics (FTEE) to satisfy high temperature resolution without strain induced signal distortion is of great significance but still challenging. Inspired by the nacre microstructure capable of restraining the stress concentration, we exemplify a versatile MXene-based thermistor elastomer sensor (TES) platform that significantly alleviates the strain interference by the biomimetic laminated strategy combining with the in-plane stress dissipation and nacre-mimetic hierarchical architecture, delivering competitive advantages of superior thermosensitivity (−1.32% °C−1), outstanding temperature resolution (~0.3 °C), and unparalleled mechanical durability (20000 folding fatigue cycles), together with considerable improvement in strain-tolerant thermosensation over commercial thermocouple in exercise scenario. By a combination of theoretical model simulation, microstructure observation, and superposed signal detection, the authors further reveal the underlying temperature and strain signal decoupling mechanism that substantiate the generality and customizability of the nacre-mimetic strategy, possessing insightful significance of fabricating FTEE for static and dynamic temperature detection.

The electrochemical properties of PVA, PTF, PTF/MXene, and PTF/MXene/Fe composites are characterized by electrochemical impedance spectroscopy (EIS) where the intercepts of EIS curves with x axis can be considered as the impedance. It is observed that the PTF/MXene/Fe composites possess the lowest impedance ( Supplementary Fig. 13a), evidencing that the MXene nanosheets played the key role in electrochemical performances. Supplementary Fig. 13b and c further present the I-V curves and current values at 1 V of PVA, PTF, PTF/MXene, and PTF/MXene/Fe composites at 20 °C and 40 °C, where PVA and PTF composites remain virtually constant. In contrast, the current values of PTF/MXene, and PTF/MXene/Fe composites dramatically increase (from 2 to 4.43, and 2.4 to 5.57, respectively), which is consistent with the increasing tendency of temperature. These findings further suggest that the thermosensation mainly ascribed to MXene nanosheets. We fabricate and examine a type of reference sensor, the homogenous elastomer sensor, which keeps the TOCNF, MXene nanosheets, and Fe contents, only eliminating layer-by-layer assembly and interlayer spraying processes. As a result, the homogeneous elastomer sensor shows resistance variation (~ 34.2) associated with response time (~ 20 s) from 20 to 40 °C, while the TES in conjunction with LBL assembly that facilitating MXene nanosheets forming thermal paths possesses obvious variation value about 39.4 with fast response time (~ 13.5 s). This phenomenon reflects that the tight packing of MXene nanosheets during the LBL process may facilitate the construction of thermally conductive pathways and reduce the energy barriers for electron hopping.

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Supplementary Fig. 15. The stable thermosensation of TES. a Temperaturedependent relative resistance curves of TES by different batches and b the corresponding temperature coefficient of resistance (TCR). c The neglectable heat transfer obstruction of the ultrathin FEP encapsulation layer (80 um) for stable thermal performances of TES. Supplementary Fig. 16. Thermal response time. The rapid thermosensation of TES to different temperature gradients from constant initial 19.6 ℃ to a 22.4 ℃, b 24.1℃, c 33 ℃, d 45 ℃, and e 70 ℃, respectively).

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Notably, the thermal resistance variations at 22.4 °C (13 s) required a much longer time than that of at 70 °C (7 s). We attributed this result to the fact that a larger temperature gradient between the heat source (70 °C) and ambient surrounding (19.6 °C) may cause a stronger thermal convection, resulting in more dramatic resistance changes and achieved the heat equilibrium in a shorter time. The relative resistance variation displayed distinct divergence between cold-hot temperature interval, and the output signals under each temperature gradient showed negligible deviation, implying high durability and stability of the TES sensors. As for the stability concern, we also investigate the time gradient influence on the reliability of TES, and the stable signal output still could be recorded and possesses 90.9% TCR value retention even over a long period of 45 days that sufficient for the most of possible application duration, corroborating the extraordinary long-term reliability of the TES.

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Supplementary Fig. 25. The strain and temperature decoupled mechanism. The Schematic illustration of the bimodal decoupled mechanism for the thermistor sensor when a the strain is applied, b a temperature gradient is applied, and c coupled strain and temperature stimuli are applied simultaneously.
Supplementary Fig. 26. The comparison of stress dispersion under external loading in the homogeneous structure, nacre-mimetic architecture, and interlocking laminated architecture.

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Supplementary Fig. 27. Photographs of puncture resistance test and the corresponding needle size parameters.
Typically, the PTF/MXene/Fe composites of sensing layer (0.60 mm) could withstand a puncture force (needle diameter = 900 um, loading speed = 50 mm min -1 ) as high as 13.42 N, which is superior to PVA, PTF, and PTF/MXene composites with the force is only 3.9, 6.7, and 7.6 N, respectively. This striking out-plane puncture tolerance further reflects the dominant role of coordination bonds in topological interlocking where the efficient stress dissipation is achieved through the interfacial bridging so as to enable excellent impact resistance. The tearing energy test was conducted by the tensile test using the single-edge notched sample (length of the slit = 1 mm). The notched and unnotched specimens (gauge length of 10 mm, width of 5 mm, thickness of 0.6 mm) were both tested at the constant stretching speed of 3 mm min -1 . The fracture energy (Gc) was calculated by the Eq.S3: where represented the length of the slit (1 mm), represented the elongation-

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at-break of the notched sample, represented the strain energy calculated by integration of the stress-strain curve of the unnotched specimen until ( = -1).
With the elongation of notched rectangular PTF/MXene/Fe composites specimen (size = 10×5 mm, stretching speed = 3 mm min -1 ), the crack gradually widens along the longitudinal direction until the sample becomes damaged at a strain of 1398 %. The fracture energy of the PTF/MXene/Fe composites is calculated to be as large as 591± 35 kJ m −2 , which is over 1.3 times higher than that of PVA (454 ± 24 kJ m −2 ), because of the presence of interfacial interlocking and dynamic cross-linking in the matrix.

Supplementary Note 1. Preparation and proposed mechanism of high crystallinity PVA polymer networks.
With respect to the film supporting skeleton, PVA is chosen to ensure structural integrity of the nanocomposite films due to its good water solubility, cost-effective, low-toxicity, and superior mechanical robustness. According to previous reports, [1] the construction of dense polymer networks and achievement of high degree of crystallinity are the major contributing factors to prepare the PVA layers with high mechanical properties and structure stability. Herein, the applying of a strong alkaline hydroxide (6 M) into a dried PVA single layer brings the twofold implications. First, OH − of the alkaline hydroxide attacks the hydroxyl groups of PVA, resulting in disrupted hydrogen bonds and deprotonation of the hydroxyl groups of the PVA chains that described as Eq. S1: Second, the complexation can be formed between Ogroup and the free Na + , facilitating the mobility of PVA chains to be aligned and formation of crystalline domains. After adding water to remove Na + ions, the Obecame protonated and crystalline domain was permanently stabilized, leading to the PVA polymer networks with high crystallinity.

Supplementary Note 2. Structural optimization towards mechanical performances.
Although the large-size PTF/MXene/Fe composites (sensing layer, 25 × 45 cm 2 ) can be fabricated via the straightforward interfacial bridging and facile LBL assembly procedure towards the commercial application ( Supplementary Fig. 10a), the basic mechanical requirements of FTEE withstanding arbitrary deformations requires further structural optimization for mechanically resilience and robustness, which weighs equal significance compared to the thermosensitivity. For this concern, the influence of the stacked layers number (1,4,8, and 16) on mechanical performance is initially discussed owing to its preliminary role in balancing the stiffness and elasticity (Supplementary is demonstrated for all counterpart samples, convincing the desirable reinforcement effect of the coordination bonds ( Supplementary Fig. 10d). In view of the skin-like Young's modulus (0.35 ± 0.032 MPa) and a relatively high toughness (50.62 ± 1.53 MJ m -3 ), the TOCNF content of 1 wt% is optimal for the subsequent context.
Moreover, the PTF/MXene/Fe composites can be easily elongated to more than 10 times its original length without fracture by uniaxial test, revealing excellent elasticity and stretchability ( Supplementary Fig. 10e and 10f).  [2][3][4][5][6] In order to dynamically manifest the stress distribution in nacre-mimetic architecture during distortion, we developed a 3D finite element model using CINEMA 4D R20 software for the nacre-like structure that duplicated the "brick-and-mortar" arrangement from mollusk shells to achieve the finite element simulation (ABAQUS).With regard to the interface of adjacent layer, we designed the layer volume network combined with random volume growth strategy to mimic the random crosslinking between adjacent layers, thus the adjacent layer possessed random and discrete interface gap as default option in ABAQUS simulation.